Internal passages are not commonly provided in products produced by a die casting process. Casting an internal passage requires the use of a core produced from a material that can be removed from the cast product without damaging the product. Sand and foam are common materials used for making cores. Sand is removed by demolishing it and foam is removed by burning. However, neither is considered suitable for die casting. The die casting process subjects the cores to high pressures during the fill process that are typically beyond that which can be withstood by sand and foam cores. Sand cores are also difficult to demolish and the destructions of foam produces toxic contaminants that are environmentally undesirable.
Accordingly, die casting is most commonly achieved without the production of internal passages or cavities. Products produced by die casting that have internal passages require follow-up processes. More often such products are produced by methods other than die casting.
An alternative material was developed some years ago as having properties desirable for producing cores for casting. This material has a salt base (soda ash or perhaps more accurately an ionic compound) that is dissolvable in water but in a dry state can be provided in powder-like form. The dry powder form can be melted into a liquid or molten state, poured into a mold and cooled to a solid state. That is, the core is produced by "casting" the desired core shape and the core produced thereby is in turn used for the casting of a metal product requiring internal passages. Such a core can be inserted into a casting mold, and following the casting process, it can be simply washed out of the product to leave the internal passages or cavities. Such a material is available from Park Chemical Company of Detroit, Mich. and the material is known as Aluminum Casting Salt. Hereafter this material is often referred to simply as casting salt but includes all materials having similar properties, e.g., an ionic compound that can be molded into a solid state and washed away with water.
The problem with this material is that the casting salt cores previously produced with any degree of success have been produced by the known method of gravity flow casting. These have been found to be too fragile for use in die casting. Whereas the die casting process as compared to gravity flow casting is known to produce a stronger casting, that has not been the case for producing cores of casting salt. Prior attempts to produce cores from casting salt have resulted in disaster. The liquid molten casting salt has such a low viscosity that the material cannot be contained in the die casting molds. When the material is converted to its molten state and injected into a casting mold under the required high pressures, the material simply blows or spits out the vent holes (or out the seams of the mold in a natural venting process). Thus, the use of casting salt for making cores for die casting has been extremely limited. It is believed that there is a single application where such cores are die cast but under extremely unsatisfactory conditions. It is understood that in this application the molten casting salt spits or blows out of the seams and the process is both wasteful and messy and requires special preparation and cautious handling.